A key factor to the growth of present cellular telephone systems is the portable telephone's convenient small size and light weight. That allows the person to carry the portable telephone in a coat pocket, which is most convenient. As a consequence of that convenience and its recognized utility, the number of portable telephone subscribers continues to grow.
Various communications satellite systems under development, such as that system referred to as Odessey, are intended to provide improved wireless digital telephone communications for ground based mobile telephone subscribers, a term encompassing those who transport the telephone on their person. In those systems, a sufficient number of satellites in medium earth orbit MEO), suitably twelve in Odessey, are to provide communication coverage over a major portion of the earth. Using a self contained battery operated digital wireless portable telephone handset to communicate with those satellite communications systems, the telephone subscriber is expected to be able to contact or be contacted by others anywhere on earth.
Because the distance from the user's handset to the RF repeating equipment in the communications satellite is much greater than the analogous distance between a cellular user's handset to the telephone cellular repeater equipment, typically found nearby atop tall buildings, and because the transmission frequencies employed in the satellite system is significantly higher, the handset equipment used in the existing cellular system, specifically the antennas, cannot retain their structure and be scaled in size for use in such satellite systems. Among other things, the change requires different antenna technology.
Due to its superior performance compared to other types of antennas, the known stacked quadrifilar helix antenna (QHA), used earlier in the INMARSAT satellite system, has been proposed by others for use in the Odessey system. The quadrifilar helix antenna is composed of four identical helixes wound, equally spaced, on a cylindrical surface. For transmitting, the helices are fed with signals equal in amplitude and 0, -90, -180, and -270 degrees in relative phase to produce circularly polarized electromagnetic radiation (RF). That antenna provides a generally hemispherical radiation pattern.
A stacked quadrifilar helix antenna incorporates two such antennas, one located over the other along the same cylindrical axis. The upper antenna serves for transmitting RF energy at one frequency and the lower antenna for receiving RF energy at another frequency, which, for Odessey, are suitably of 1.618 GHz and 2.483 GHz, respectively, in the microwave frequency range.
Owing to the possible small size, light weight and circular polarization properties, apart from its feed network, that quadrifilar helix antenna appears attractive. However the antenna's helices are fed microwave energy by either a quadrature coupler or by a balun. A balun is a passive RF matching device that converts a transmission line carrying the transmit and/or receive signals, such as a coaxial cable, strip line or microstrip and the like, into a balanced feeder. At microwave frequencies, resonant transmission lengths in baluns act as wave traps and incorporated feed phase inverters. It is an equal power divider, in the case of transmitted microwaves, and an equal power combiner in the case of received microwaves, having perfect return loss at the input, no matter what kind of electrical impedance appears at the outputs. Although the foregoing balun feed network properly functions in the antenna combination, proving the technology's worth, a significant practical disadvantage to the present is that the feed system is large in physical size, larger than the size of the electronic equipment in the handset. The quadrature coupler also has that disadvantage.
Consequently, if used on a handset communicator, the handset unit could not be conveniently stowed in ones coat pocket, and, as a consequence, the commercial viability of the communication system appears implicitly threatened by a foreseeable inability to attract the interest of sufficient numbers of consumers to carry about an awkward device. That concern provides incentive for an antenna system that is small enough or an antenna feed that is small enough in size that makes practical a communicator package or handset that may be carried within ones coat pocket.
An object of the present invention, therefore, is to realize an antenna package for a handheld portable battery operated communication transceiver, a wireless telephone, into a small size, small enough to be "consumer friendly", allowing the consumer to conveniently carry the telephone on the person. An ancillary object is to provide the foregoing at low manufacturing cost while maintaining acceptable levels of RF performance. An additional object of the invention is to provide a new geometry for a microstrip balun structure. The present invention realizes those objectives.